Methods in which propylene, isobutylene or tertiary butyl alcohol is used as a raw material to produce a corresponding unsaturated aldehyde or unsaturated carboxylic acid are industrially widely carried out. However, the generation of a local high-temperature portion (hot spot) in a catalyst layer is a significant problem. The generation of a hot spot leads to shortening of a catalyst service life or lowering of a yield to be caused due to an excessive oxidation reaction, and in the worst case, the generation of an accident hazard due to a runaway reaction or out of order of a catalyst, and therefore, technologies for suppressing the hot spot are proposed. For example, Patent Document 1 discloses a technology of using a shaped catalyst whose activity is regulated by varying an occupation volume of the catalyst or a calcining temperature of the catalyst.
Among such catalysts, with respect to those having a large average particle diameter and/or those having a high calcining temperature, the catalyst layer becomes thick, and therefore, there may be the case where a strain in the catalytic active component layer is generated, or the mechanical strength is lowered due to a crystal phase change at the time of calcination, and there is a concern about the generation of such a problem that during the storage of a completed catalyst, the catalyst in a storage container bottom is broken, or at the time of filling in a reaction tube, the catalyst is broken, resulting in an increase of pressure loss of the reaction tube. Above all, such a tendency is more likely seen in catalysts having not only a large average particle diameter but also a high calcining temperature, and the production efficiency of the catalyst is remarkably lowered, and hence, improvements are considered to be needed. It is to be noted that the calcimining temperature as referred to herein indicates a maximum temperature in a calcining step to be carried out for the purpose of imparting activity to the catalyst, and typically, it means a maximum temperature of the temperature of calcining or drying to be carried out for the catalytic active component.
As a method of enhancing the strength of a catalyst, Patent Document 2 discloses a shaped catalyst by containing inorganic fibers in a ring-formed shaped catalyst containing molybdenum and bismuth. In addition, Patent Document 2 discloses that at least one member selected from glass fibers, aluminum fibers and silica fibers, each having an average fiber length of 50 μm to 1.5 mm and an average diameter of 2 μm to 20 μm, can be used as the inorganic fibers. However, though the mechanical strength is improved to some extent by adding such a shaping auxiliary agent, the yield of the target unsaturated aldehyde or unsaturated carboxylic acid is not sufficient yet, and catalysts having both mechanical strength and catalytic performances (for example, activity, yield, etc.) are considered to be needed.
Patent Document 3 discloses a method of using, as a carrier assistant, inorganic fibers having an average particle diameter of 2 to 200 μm as a method for improving of the mechanical strength of a supported catalyst.
Patent Document 4 discloses a method of adding a silica sol and inorganic fibers. Patent Document 5 discloses a catalyst containing a scaly inorganic material having an average particle diameter of 10 μm to 2 mm and an average thickness of 0.005 to 0.3 times the average particle diameter. Patent Document 6 discloses a catalyst containing inorganic fibers having an acid amount of 0.05 mmol or less. It is mentioned that by controlling the acid amount of the inorganic fibers to 0.05 mmol or less, a catalyst of a high yield can be obtained.